1. Field of the Invention
The present invention relates to the use of electrical resistance tomography (ERT) to determine subsurface soil or rock temperatures. More specifically, it relates to a method for mapping temperature changes in subsurface soils through remote measurements.
2. Description of Related Art
Many applications require heating of subsurface soils or rocks. For example, steam injection can be used to heat soils and remove subsurface contaminants such as Volatile organic compounds. Steam injection is also used in oil reservoirs to enhance the recovery of petroleum. In both of these applications it is highly desirable to map the subsurface region through which steam has moved and to determine which regions have been bypassed by the steam. These processes can be made more cost-effective when the temperature distribution in the region of interest is known. Subsurface temperature measurements are generally made from boreholes using sensors which sample temperatures at a point along the borehole wall, however, these measurements only represent the temperature of the soil located within inches of the borehole. In order to measure temperatures at locations removed from one borehole requires drilling additional boreholes and the regions between boreholes cannot be measured. It would be beneficial to have a method which would minimize the need for and increase the utility of boreholes by providing means to monitor temperature changes remotely using electrodes located at the ground surface or in boreholes. The present invention provides such a method.
For a description of a simple physical model describing shaly sand conductivities, see M. H. Waxman et al., "Electrical Conductivities in Shaly Sands-I. The relation between hydrocarbon saturation and resistivity index; II. The temperature coefficient of electrical conductivity", Journal of Petroleum Technology, Vol. 26, pp. 213-218, 1974.